text:
What would happen if you turn on all air vents simultaneously?
Sitting on the plane yesterday, wanting some fresh air and boom it hit me [literally]. What would happen, let's say both static and mid flight, if everyone turned on there air vents at the same time?
Could the unit handle all of it simultaneously, would it effect pressure, I'm not privy to the working of air units of aircraft so it may be a silly question but it's still a question that I'm interested in knowing the answer to.

Absolutely nothing will happen. The vents have no effect on the air cycle machines or circulation fans.

@Casey is correct. Nothing will happen under normal operating conditions. The FAR §25.831 Ventilation requires,

... the ventilation system must be designed to provide each occupant with an airflow containing at least 0.55 pounds of fresh air per minute.

In general, the amount of air available is far greater than the requirement. On the ground, compressed air for the Environmental Control System, which supplies air to crew and passengers can be obtained from an APU, a GCU, airport high-pressure hydrants, or the aircraft engines. In flight, however, compressed air is obtained almost exclusively from the compressor stages of the aircraft engines.
The pressure would not be affected by this. According to FAR §25.843, Tests for pressurized cabins, the pressurized cabins are tested to show the performance of

... the pressure supply, pressure and flow regulators, indicators, and warning signals, in steady and stepped climbs and descents at rates corresponding to the maximum attainable within the operating limitations of the airplane, up to the maximum altitude for which certification is requested.

Note that in some aircraft the air conditioning system is switched off during climb- this is to do with the load on engine and not the air supply.

New aircraft designs may also use an independent compressed air source rather than the engines. This is the case of the B787 where Boeing switched from engine air to electrically-powered pressure supply to increase engines efficiency (and possibly air quality) and decrease weight and pneumatic system complexity.

To add to @aeroalias' excellent answer: compressed outside air (whether from the engines or separate compressors) is fed into the plane's air conditioning system, from where it's distributed to the cabin, the cockpit, etc. The air destined for the cabin is routed to air outlets (sometimes called diffusers), installed above and/or below the hatracks (picture - look for some long, thin openings near the hatracks next time you fly), which blow the air into the cabin. Air is blown into the cabin to control the temperature in the cabin and to ensure a constant supply of fresh air for the passengers.
The air gaspers (what you call "air vents") above the passengers are (usually) connected to the same ducts which feed the air outlets, which means that when you open a gasper, a tiny amount of the air which would normally enter the cabin through an air outlet enters it through the open gasper instead. Therefore, even if everyone opened their gaspers, the total amount of air going to the cabin wouldn't change - except that some of it would be routed via the gaspers instead of the "normal" air outlets. However, even in this case, the majority of the air would still be routed to the air outlets.
(Note that when I say "routed", it's all done passively via the design of the air conditioning ducts and manifolds; there is no "active" routing (via valves or suchlike) between the air outlets and the gaspers.)


text:
What are the flying accommodations in Silicon Valley for private pilots?
What are the flying accommodations in Silicon Valley for private pilots? I notice there are two pretty sizable airports: Moffett Airfield and Mineta San Jose. Do both accept GA flights and have landing fees below $50?
What about instruction? Where do pilots generally go for instruction in the San Jose-Palo Alto area?

@zach-lipton has a pretty concise answer about airports.
Due to the SFO class B airspace, the training area is primarily NE (Calaveras) to SE ("UTC" / Anderson Lake) of KRHV (and South to East of KLVK). If you're departing from KPAO, it's a good 15-20 mins each way to and from the training area. KPAO also has IMC more frequently than KRHV, which is a big advantage if you're mostly going to fly VFR. It's also further out to get to non-towered airports from KPAO (E16/KTCY/C83 are the closest). Highway 101 N/S is pretty busy around KPAO (Embarcadero exit), so unless you will generally avoid the morning/evening commute, KRHV may be easier to get to by road than KPAO.
KSJC is a busy airport, but they do accommodate GA training operations if they can. I would recommend against using KSJC GA as your transit point though -- KRHV is about 5 miles away, and much cheaper.
If you're a visiting pilot and want to use the transient parking, transient tiedown fees apply -- see KPAO, KRHV. Some FBOs or flight schools may offer lower tiedowns in their ramp areas at the airports.

Moffett Field is still primarily a Federal airfield. They'll take some corporate jets, but not GA, and prior permission is required. All requests go through Planetary Ventures LLC, which owned by Google.
San Jose is a large international airport, but it appears to have no landing fee, though you'll pay a ramp fee unless it's waived. Some AirNav reviews of the FBO give you an idea of what to expect, and you could contact them for further information.
There are a number of GA airports to choose from though. Including:

Reid-Hillview
Palo Alto
San Carlos

All of these airports have flying clubs and should offer instruction.
Along with others somewhat further away (a few examples include Half Moon Bay to the Northwest, San Martin to the south, Hayward Executive and Oakland International to the east), depending on where exactly you'd like to be.

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text:
Is a clearance needed to depart a helipad in class B airspace?
I have noticed statements like the following on sectionals:

This would seem to suggest that a clearance is needed to "depart" any helipad, even if you are staying under the wedding cake. Is that true?

The operative words are "within the Class B Airspace", not under it.

It likely depends on country, are you asking for China?

@abelenky Yes, I guess I was just misinterpreting it.

See 14 CFR 91.131

The "Wedding Cake" literally defines the airspace, so for a the sectional part you listed above, the floor of the Class-B is at 3000 feet in the outer ring and 2000 feet in the inner ring, these heights are MSL.
Anything outside of the wedding cake is not Class B airspace. You may be confusing the Class B airspace with the Mode-C Veil that surrounds 30nm outside of most Class B airports, which only requires that you have an operating transponder, you are still allowed to fly around VFR squawking 1200 below the Class-B without any clearance.
The airspace under the wedding cake, provided you are not within a Class-E airport, is most likely Class-G. So no, you are not required to contact Tower until you want to enter Class-B airspace or if you need a departure clearance for IFR operations.

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text:
What does "radar contact" mean in ATC calls?
When ATC mention 'radar contact', what does that mean?
In what conditions ATC might contact you and mention 'radar contact' in the call? and what would be the appropriate responses?

The FAA's Pilot/Controller Glossary gives both US and ICAO definitions:

RADAR CONTACT- a. Used by ATC to inform an aircraft that it is identified on the radar display and radar flight following will be
  provided until radar identification is terminated. Radar service may
  also be provided within the limits of necessity and capability. When a
  pilot is informed of “radar contact,” he/she automatically
  discontinues reporting over compulsory reporting points. [...]
b. The term used to inform the controller that the aircraft is
  identified and approval is granted for the aircraft to enter the
  receiving controllers airspace. (See ICAO term RADAR CONTACT.)
RADAR CONTACT [ICAO]- The situation which exists when the radar blip
  or radar position symbol of a particular aircraft is seen and
  identified on a radar display.
RADAR CONTACT LOST- Used by ATC to inform a pilot that radar data used to determine the aircraft’s position is no longer being
  received, or is no longer reliable and radar service is no longer
  being provided. The loss may be attributed to several factors
  including the aircraft merging with weather or ground clutter, the
  aircraft operating below radar line of sight coverage, the aircraft
  entering an area of poor radar return, failure of the aircraft
  transponder, or failure of the ground radar equipment.

Note that this is not a universal term, Canada uses "radar identified" instead. Their equivalent of the P/CG is the Glossary for Pilots and Air Traffic Services Personnel:

“Radar identified”
An expression used by ATC to inform the pilot of an aircraft when radar identification is established.
Fr: « Identifié radar »

And:

“Radar contact”
U.S.: Expression for: “Radar identified”

No specific response is typically necessary (although acknowledging that you heard the transmission with a callsign or "roger" is appropriate).  The call means that the controller sees your aircraft on radar & has confirmed that "this" target is "that" aircraft, i.e. your aircraft.  He is now capable of providing ATC services that require the use of radar, such as radar vectors, separation, traffic calls, minimum altitude monitoring, etc.  Not saying that he necessarily IS providing all of those things; there's a little bit of context to such things.
You can also hear, "radar contact lost" either when you are below the radar horizon due to terrain, or over the water too far away from from the radar site.  This means that you're now, if flying IFR, operating under non-radar IFR rules, and things like making position reports at the mandatory reporting points now applies.  (You don't have to make those reports while in radar contract.)
Interestingly, you may also (rarely) hear a military fighter call "radar contact" in response to a traffic call, meaning that he sees the traffic on radar, but not visually.  Not sure if that's standard terminology for them in an ATC environment or not, but I've heard it a few times.

Yep, if we get a traffic call and have operational radar, then the first thing we'll do is attempt to lock up the potential contact, check its BRA, and then report back "radar" to ATC.

We're not all @RhinoDriver 's, can you explain "check its BRA" - kinda sounds like a fraternity stunt...

@FreeMan BRA is a brevity code which stands for: bearing, range, altitude.

"Radar Contact" is followed by a quick description, such as "Radar Contact, 15 south of McCarren, climbing through 2000".  This is your opportunity to disagree if you think they might've picked the wrong radar target.  You could say, "Negative:  I'm at 6000 and descending."  (It is admittedly, extremely rare for ATC to report radar contact when they aren't completely sure about which blip is which target).

Context: You will typically get this the first time you contact an ATC facility and they reply with a squawk code.  "Radar contact" is to tell you that they now know which blip (aka contact) on their radar display is you and are thus able to provide various radar services--though whether they actually will is another matter.
Notably, if you don't hear this, you're likely either below their radar coverage or talking to a facility that doesn't have radar at all.  Obviously, they're not able to provide radar services in that case.  They're likely to ask for position and altitude next so they know where you are relative to other aircraft.  If they do see a primary radar return at the position you give them but no secondary, it's likely your transponder isn't working; in that case, they probably won't ask position again but will ask your altitude occasionally--and near a busy class B/C airspace, will likely tell you to keep out because having to verify that every time another plane comes near adds too much work.
Responses: If all they say is "radar contact" and nothing else, just acknowledge it with your callsign.  Usually, though, they'll start giving you traffic advisories, and you'd reply to those with "looking" or "in sight".  If you didn't specify why you're calling them on your callup, they may also ask you to "say intentions" so they know what to do with you--and may start giving you vectors accordingly, which you would need to repeat back as usual.

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text:
How can a computer model yield two possible flight paths of MH370 in South Indian Ocean?
I am reading this article on the NY Times, where they show two possible flight paths of the missing flight MH370 in the South Indian Ocean:

I am wondering how they can come up with such specific paths, when the only information we have are those two arcs (those arcs are not paths, rather the last satellite ping happened somewhere on the arc).
How can anybody create a computer model with just this information? Or does it mean that the modelers have more information available than the general public?

http://online.wsj.com/news/articles/SB10001424052702304017604579443603302188102

how they can come up with such specific paths?

Diagrams in news organisations graphics published in March and April 2014 vary widely in what they show and appear to be artistic impressions. These should not be relied on in any way.
To calculate a variety of likely flight paths, it seems Inmarsat engineers, and other parties to the official investigation, used a number of sets of information including but not limited to:

satellite communication timings giving distances from satellite.
satellite communications frequency variations giving relative motions.
knowledge of variations in satellite orbit.
knowledge of variations in satellite temperatures (daylight exposure etc)
knowledge about signal processing timings in various equipment.
characteristics of the aircraft (load, fuel consumption rates, etc)
autopilot modes of operation (route, heading, track. magnetic, true, ...)
etc.

The analysis was improved and refined over time, resulting in newer estimates of the flight path and final coordinates.
Other independent groups of people have published their own analyses which come to broadly similar conclusions. E.g. the self-titled "Independent Group" associated with Duncan Steel

Inmarsat paper, October 7 2014.
A paper "The Search for MH370" published in the Journal of Navigation by Ashton et al. (2014). describes in some detail how the Inmarsat engineers used satellite data to determine probable flight paths for the aircraft.

ATSB report. August 18 2014
The Australian Transport Safety Bureau released a revised version of their June 26 report.

ATSB report. June 26 2014
The Australian Transport Safety Bureau release a 54-page report "MH370 - Definition of
Underwater Search Areas" which goes into greater detail than any prior official report and shows much more of the calculations that were performed on the Burst Frequency Offsets to calculate probable final position of MH370.
In particular the appendices describe some  equations that were used.

A small extract from the report

Raw Satellite Data. May 27 2014
The raw satellite data was made available on 27/5/2014 in response to requests from relatives of the passengers. This shows burst timing offsets (BTO) and Burst Frequency Offsets (BFO) for communications between the Inmarsat ground station and MH370.

ATSB document. May 26 2014.
The Australian ATSB released a factsheet that outlines what is known and reiterates the general description of how the tracks were calculated:

By
  taking the maximum speed of the aircraft into account, the rings can be reduced in length to arcs – there
  are some areas of the rings it simply could not have reached.
There are a large number of speeds and headings that can be
  consistent with a BFO recording. These are limited, however, by the operational constraints of the aircraft.
  Candidate paths of different speeds were created which met the BTO ring location/time constraints and
  the predicted BFO values of these paths have been compared with the recorded values. The better the
  match, the higher the probability that the path was close to that of MH370.

Release of raw data from satellites. May 27th 2014
On May 21st 2014 CNN reported that the satellite data and description of analysis would be published. No date has been specified.

"In line with our commitment towards greater transparency, all parties are working for the release of the data communication logs and the technical description of the analysis for public consumption," Inmarsat and the Malaysian aviation officials said in a joint statement.

Inmarsat Statement. March 24 2014
On March 24th 2014, Inmarsat stated that a new analysis of the records of satellite handshakes with MH370 were able to use doppler effects in the signals together with an analysis of similar satellite handshake data from other Malaysian 777s to determine that MH370 flew into the Indian Ocean and ruled out flight paths to the previously described northern corridor.

From INFORMATION PROVIDED TO MH370 INVESTIGATION BY UK AIR ACCIDENTS INVESTIGATION BRANCH (AAIB)
25/03/14

Malaysian Defence Minister statement. March 21 2014.
On March 21st, a little light was shed on this process by a statement attributed by New Straights Times to Malaysian Defence Minister and acting Transport Minister Datuk Seri Hishammuddin Hussein.

"The investigations team received the complete raw Inmarsat satellite data, which included the six handshakes at approximately 3 pm on Wednesday, March 12."
"This type of data is not normally used in investigations of this sort.
  It is only because we have so little other information to go on in this difficult and unprecedented situation that the data is being used.
"Upon receiving the raw data, the Malaysian authorities immediately discussed with the US team how this information might be used. The US team and the investigations team then sent the data to the US, where further processing was needed before it could be used.
"Initial results were received on Thursday, March 13 at approximately 1.30 pm, but it was agreed by the US team and the investigations team that further refinement was needed, so the data was again sent back to the US.
"The results were received at approximately 2.30 pm on Friday, March 14 and presented to the investigations team at a high-level meeting at 9 pm on Friday.
  The UK AAIB, who had also been processing this data independently, presented their results - which concurred with ours and those of the US team – at that meeting.
"The Prime Minister was briefed on this satellite information at 8 am Saturday, March 15 and publicly announced it at the press conference at Saturday lunchtime.

(my emphasis)
Obviously the above does not describe the actual calculations involved. However it does confirm that the satellite data had six handshakes (presumably at hourly intervals up to 08:11).

Early speculation
The graphic below, illustrates one idea for how these track might have been deduced. It is a result of speculation rather than a reporting of information actually provided by the people who made the calculation.
The graphic suggests that the NTSB calculated possible paths based on data that is not public and that the NTSB were able to assign probabilities to those paths. As far as I know, neither Inmarsat, the NTSB nor the Australian government (or it's agencies) have made public the data or method of calculating these tracks.
The graphic below suggests several Inmarsat pings were used (the example shows 01:11, 02:11, 04:11, 05:11, 06:11, 07:11 and 08:11 but these are probably speculation).
So far as is publicly known, there was no ping at 09:11 or later which suggests the flight ended between 08:11 and 09:11. This is broadly consistent with maximum fuel range of the aircraft. The labelled NTSB solutions suggest that NTSB calculated that no other constant speed (and heading?) tracks fit the data.

source Scott Henderson
which says ...

For a constant speed track, distance between ping arc intersections will be equal.
NTSB solutions are based on constant speed straight track. Curve in track shown due to map projection

A later version of this graphic adds a disclaimer saying that the tracks and zones are not accurate and are symbolic only.
We do know that NTSB sent staff to Malaysia to assist in the search. We know that Inmarsat provided data to the investigating teams.

These tracks have no credibility unless you apply selective logic ignoring aspects which can't be reconciled. The satellite BOF track does not match the early flight observed by radar at all except by "bent" logic. Once the hypothetical turn south is made past Sumatra satellite data shows velocity towards the Satellite whilst the track infers velocity away. This explanation serves to explain the incantations of a voodoo belief system, but has no relationship to MH370. More taxpayer money about to be wasted to stroke certain egos.

In their Flight Path Analysis Update of 8 October 2014, the ATSB explain how they used the BFO data to identify a range of times for a turn south, consistent with the primary radar track associated with MH370. See "Refinement of southern turn location using ground initiated phone
call data" on page 8.

The "related article" specified in your link has this to say:

Searching one possible flight path, Australia said it would focus its
  efforts in a specific stretch of the southern Indian Ocean, using
  computer models of the plane’s possible flight path that take into
  account undisclosed satellite data, wind conditions and ocean currents
  and some assumptions about how fast it was flying and how much fuel it
  had left.

So yes, apparently they have more information than the general public does.

My personal guess: I think what they've done is to start a line where from where the
   plane was last known, and then intersected it with the arc where the
   distance would have been correct at cruise speed with reference to the time.
Example with just some values. Since the arc radius and time was known, one would be able to draw a line which would show a constant speed. If it was not a constant speed, the line would have to bend left/right to account for it, which does not appear to be the case in the provided image.

This is based upon the following:

'A signal was sent out every hour'. In other words, they got several data points.
Based upon that, they would have points for the arc radius at set times.
They might assume that the aircraft has a constant speed. Because of this, they should see if the data points lie on the a straight time, since the only variable is the distance.

I'm pretty sure that the two lines is based upon the fact that they know the uncertainty at the final position, and in my opinion it suggests that the aircraft flew a constant course from the last known radar fix where it originates.

The only problem with this approach is that it assumes that the aircraft flew straight the entire time, and we have no reason to believe that this is the case.

and how do you explain that they have two possible paths, which seem to almost converge at the top ?

even if you have one fixed point where the plane was last seen by radar, how do you know which direction to draw the straight line to intersect the arc? There are infinitely many such solutions. You need some boundary conditions.

@MartinVegter imho, the increase in the distance increase between the lines from malaysia is pretty constant if you were to extrapolate the line. The difference might be caused by the uncertainty in the satellite instruments where it was last seen.

@user1968963 yes, of course, but the article states that there was a signal every hour, so they should be able to estimate the speed, and with the time to distance to get the intersection point :)

@Lnafziger since the article describes the aircraft sent out a signal every hour, they should be able to estimate the direction and speed. i'm not sure if there's doppler shift, but i think they might be able to see a linear behaviour between arc radius and speed, and the path.

Whilst I fully get the logic you have described and described very well thank you, you also hit the nail on the head that it depends upon having the correct last known location for MH370. A so called military radar image by Malaysia which claims to show the track of MH370 through the Straits of Malacca from Butterworth is a fake. The actual radar is a Thales Raytheon GM400 which has a screen with a black background, not amber. The image people rely on is actually a screen from a civilian SSR radar which could not track an aircraft without a transponder. Hoax

I provided a potential mathematical solution on deriving the flight paths using the ACARS data on 17 March (before the possible flight paths were published by the authorities) on CNN's iReport: http://ireport.cnn.com/docs/DOC-1108312

The key to finding MH370 without the aid of the transponder data but with regular attempted ‘handshakes’ from the ACARS satellite is by a process known as “vector addition”. If the last handshake with the ACARS satellite took place at 8.11am as reported then depending on the frequency of the previously attempted handshakes (half hourly or hourly, there seems to be some discrepancy in the reported frequency) a set of flight route models can be easily constructed giving a high probably of the final location based on the accurate last known location at 2.40am. Therefore, depending on the frequency of the ACARS handshakes there should be at least 6 and up to 12 vectors that could be mathematically solved.
Assuming a constant speed for the first probability model, each time the ACARS handshake occurs it gives only a footprint angle on the Earth (as has been shown on the reported 8.11am ACARS ping map; 40 degrees). The next time it handshakes another angle is indicated. At a constant speed only two possible headings are possible to ‘join’ the ACARS isogons (for the plane to get from one ‘angle’ to another). If MH370 was heading in generally the same direction or arcing across the sky, (even if there was some tacking back and forth like a yacht) only one of the possible vector solutions will be valid.
If the vectors from the last known location at 2.40am were systematically added together the final solution at 8.11am will bisect one of the two arcs provided by the authorities. Subsequent models at different average speeds and taking into account routes that would more effectively avoid radar detection would refine the models to a few high probability scenarios and narrow down the search area considerably.
It’s simple mathematics but there is no indication the authorities are using it.

As RedGrittyBrick has pointed out this is almost definitely how the flight path was estimated. The reason for the two flight paths is probably illustrating the two end member solutions based on the accumulated error using this technique (the more times you add one vector to another the more error can be introduced). Hence the two predicted flight paths diverge.
The reason for the confidence by the authorities in the southern Indian Ocean route is that the mirror set of solutions would need MH370 to pass straight over India and Pakistan; and they have emphatically denied this could have taken place.
Therefore there is the highest probability that MH370 ended up in the region of the southern Indian Ocean they are currently searching.
I am somewhat saddened that these probability models were not derived within days MH370 went missing and that at least 7 days were wasted searching in other areas.

The short answer is they don't know these are the final tracks for MH370 at all.
The two lines more likely represent a range of probability between them defined either by variablities in aircraft cruise speed or variabilities from cumulative calculation errors.
Plotting these paths with such certainty requires the last known position of MH370 to be known with certainty, but it isn't.
Whilst the "vector addition" method is not the same as another navigation practice known as "dead reckoning" there are striking similarities such as the amplification of track errors. The entire process is founded on a string of assumptions all of which have to be correct for the final solution to work.
There is good reason to question the credibility of these assumptions:

The problem with this carefully plotted track is it assumes the last position of MH370 above Aceh at 2:22am is known with certainty. That is not the case at all. Malaysian evidence for MH370's last position is increasingly discredited. It is even questionable whether MH370 was in the Straits of Malacca at all?

For example the alleged Butterworth radar image does not show the tracks of two other aircraft known to be in the same area at the same time. Why not?
Another credibility problem is that Malaysia says the image which it showed to relatives at the Lido hotel on 21 March was from a military radar at Butterworth. In reality that radar is a Thales Raytheon GM400. This image is from a real Thales Raytheon GM400 display screen:

The alleged radar image which satellite track plotters have relied upon is this from the Lido hotel on 21 March:

The alleged radar image from the Lido hotel is actually the screen-shot taken from a civilian SSR radar. That being so it could not be an image of MH370 because as we are told ad nauseum, is that MH370's transponder was not working. No SSR radar could have tracked an airliner without a transponder.
The credibility of Malaysia's radar image is so low that it is not unreasonable to call the image a hoax. They can't accurately plot the track of MH370 into the Indian Ocean based upon questionable radar claims like these.
Sadly another $90 Million taxpayer dollars will be wasted before people dare to question the Voodoo theology behind these calculations.


text:
What could cause a noise like a hammer on an A320 in flight?
Edited question:
I was on an A320, asleep, around the mid-point of the flight at about 30,000ft. I suddenly heard a constant noise like someone hitting the plane from outside with a hammer. The noise wasn't too loud but it was distinct. I looked around, none of the passengers seemed perturbed.
The noise continued for 10-15 minutes, stopped, then started again after another 30 minutes. I was about to bring it to the attention of the crew, but the plane had started its descent by then.
Does anyone know what the noise could have been? And should I have alerted the crew?
Edit: Checked the linked questions. Its not a PTU, because the noises were heard mid-flight not during take-off or landing. Also, the second part of the question, can be answered.

This could have been the PTU. See related questions here and here.

Did you look outside to see who that idiot was ruining your sleep?

It wasn't a colonial woman churning butter was it?

Problem: Noise coming from under instrument panel. Sounds like a midget pounding on something with a hammer. Solution: Took hammer away from midget.

We have a general question about what issues passengers can look out for, presumably a strange noise is something that the the crew would notice and not need passengers to tell them about.

@fooot When reading your proposal, Radu094's avatar popped into my mind. I just love it. Perhaps he was around… ;-)

The PTU isn't just used during take off or landing! It kicks in whenever the hydraulic lines need to balance out their pressure, no matter what phase of flight.

@JustSid Right. That's what the PTU is designed for (as I learned here :-)). But wouldn't it make the typical “barking dog” sound the A320 is famous for?

Don't worry, gremlins can't survive below FL 290.


text:
Pressure Distribution Pattern around Wing
it is clear that the overall integrated pressure on the top of the wing is less than the overall pressure at the bottom of a wing in order to produce a net nonzero lift force. Based on the attached figure below (AoA going from small to large), it seems that, for low and medium angles of attack, the pressure both at the top and bottom surfaces of the wing is lower than the free stream atmospheric pressure. The arrows' directions, from what I understand, are pressure gradients pointing toward higher pressure area, correct?
When the angle of attack is sufficiently large, the pressure on the wing's bottom surface is higher than the free stream pressure (positive pressure coefficient)...Is that correct?
Whenever the local pressure is lower than atmospheric pressure, I think air would rush to that area so air would be moving from far above the wing towards the wing top surface (same for bottom wing surface when the pressure coefficient is negative). Is that what happens?
Thanks.

You may find the website How it flies interesting.

"it is clear that the overall integrated pressure on the top of the wing is less than the overall pressure at the bottom of a wing in order to produce a net nonzero lift force." This does not take into account that the wing produces downwash at the rear of the wing to press the air flowing across the wing down, and this downward pressure pushes the wing up. The flow across the wing gets less and less smooth as the AoA increases, up to the point where the wing stalls from the loss of downwash/loss of lift.

@CrossRoads: When looking from the point of view of solving the Navier-Stokes equations, the downwash is the inevitable effect of the pressure distribution on the wings. So "take into account the downwash produced by the wing" is actually the same as "overall integrated pressure on top is less than bottom". You can design a wing where the overall integrated pressure is equal and you will find zero downwash or one where it is more and you will find upwash.

Last question first.  Yes, air does try to "rush" from high to low pressure, but the movement of the wing creates a condition where the net movement of the air relative to the wing will be downwards and backwards, and lower pressure remains above the wing. This is why airspeed is so important.
Understanding the arrows in the diagrams is a bit like reading music or another language.  They are simply pointing from lower to higher pressures.  Isobars on the weather map show the same thing as pressure gradients.
Also important is to know that you are looking at a fully symmetrical airfoil.  There will be equally opposing lower pressure areas top and bottom at 0 angle of attack.  This is why many airfoils have a flat or even concave bottom to help create higher pressure underneath the wing.  But even a fully symmetrical airfoil will have a pressure differential as AoA increases.

Yes indeed, there is also some suction (so lower than atmospheric static pressure) on the lower side, because there, the air flow is "pushed away" as well. In the first image, you can see that both pressure distributions are equal, so this symmetric airfoil doesn't produce any lift at zero angle of attack (AOA). Only when you introduce an AOA, the air "sees a more convex curvature" (so even higher pressure) on the upper side and the opposite on the lower side, which then creates some lift. The third image is quite interesting: there you actually get higher than atmospheric pressure near the nose and again lower pressure further back. This all has to do with how the streamlines bend around the airfoil:

You get higher than atmospheric pressure in all three images, at the stagnation point.

Thank you. Previously, I thought that the pressure on the bottom surface was larger than atmospheric pressure for any AoA. In reality, the pressure at the bottom surface is higher than the one at the top surface, even when both pressures are lower that ambient pressure far from the wing. If the wing was stationary and the air moved, the pressure in the airstream far from the wing would be lower than the pressure of air that is at rest. We call "static" pressure the pressure when it is measured in a flow in the direction perpendicular to the direction of airflow. That seems misleading...

Exactly, it is the pressure difference which creates lift. But be careful, it doesn't matter whether the wing or the air is stationary, as long as the relative velocity is the same. Also, the definition of static pressure is not related to how it is measured. It is simply the "actual pressure of the fluid, which is associated not with its motion but with its state" (see Wikipedia). It is usually measured with static pressure taps which are indeed perpendicular to the surface, but this is just to prevent a measurement error due to air entering the tap and slowing down.

Thanks Daniel. The word "static" has led me to believe it would be referred to the pressure exerted by air not moving. If a fluid moves to the right at speed v, the pressure measured by an vertical area placed in the fluid would be P+dynamic pressure. But if we measure pressure in a direction normal to the fluid (using the pressure taps) the pressure would be different and smaller...

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Can the Kawasaki T-4 carry weapons?
The Kawasaki T-4 is a jet trainer aircraft. However, it seems to have two hardpoints.

Armament
Two hardpoints for external fuel tanks

Can these be used to carry weapons? E.g., unguided bombs or unguided missiles?
I suppose guided missiles require avionics for a more sophisticated fire-control system, fire-control radar, and similar — which I suppose the T-4 definitely does not have. However, please correct me if I'm wrong.

Not much of an answer, but from what I've seen in my line of work, militaries will explore such options, hacking (both hardware and avionics) wherever they can even if they had bought the platform from a foreign OEM. None of these info will ever become public, if their security and intelligence agencies are any good. If you can think of it, they would have considered it as well. Viability is a completely different story though.

The Kawasaki T-4 actually has three hardpoints that can carry weapons
: one underfuselage and two wingpoints (also confirmed here). The possible armament includes gun pods, long-range anti-ship missiles, air-to-air missiles, free-fall or retarded bombs and pods with 70 mm unguided rockets. Hence no way it could be said it cannot be armed.
This aircraft seems more for training but can be used in light attacks.

Seems like someone should revise the Wikipedia article! Good answer.


text:
How much is a QF-16 worth?
Wikipedia lists the prices for F-16 as:

F-16A/B: US\$14.6 million (1998)[4]
F-16C/D: US\$18.8 million (1998)[4]

However, this is clearly outdated, and I suspect that the aircraft in the worst conditions are being used for these conversions.
It would be interesting to see a ballpark figure out the actual worth of these drones, excluding and including the conversion process.

This has a pretty broad range of answers for this, but I assume what you are asking is what a flight ready QF-16 would sell for should it become instantly available on the civilian market.
Well the short answer is whatever someone is willing to pay for it.
The long answer is that aircraft values are based on a large range of variable, including, but not limited to, the value of the airframe, the current time and status of all life limited parts, currency of all required inspections, status of airworthiness directive compliance, avionics, outstanding repairs and overhauls, etc.
Now assuming this QF-16 is an acquired F-16A reclaimed from AMARC, with a reasonable amount of time left on the airframe, engine and all functioning avionics and systems as manufactured in the early 1980s by GD Fort Worth, the jet could easily command 15 million in FY2019 USD.  Now the QF-16 conversion costs vary depending on the contract signed but typically go for about 1.1 million per jet.  Add to that a dealer and/or brokerage fees of 15% sale price, the cost of flight training a reasonable budget might be 50 or so hours at $25,000 per flight hour or 1.25 million, this gives you an out the door price in the neighborhood of 19.6 million - that is if Congress will sell it to you.


text:
Why do non-aerobatic aircraft have a negative G limit?
Why do airplane wings have limit load of both positive and negative g's (e.g., the Weedhopper ultralight has a load factor of +5 and -2gs). Why not just have a positive limit load since most airplanes (like the Weedhopper ultralight) don't fly upside down? In particular, why the negative load factors?

The aircraft has a negative g limit load, whether or not you intend to "experience" it. If you are going rely on a machine to stay alive, would you rather know what the limit is, or not know?

You can get negative load factors (g forces) in different ways than just flying upside down:

Change in pitch: When you push on the control column, the pitch will start to decrease. Depending on how fast you do this, the load factor can even become negative from this. Some aircraft do this intentionally to reduce the g force to exactly zero:

(image source: Wikimedia)
By pushing the control column further forwards, you would get negative gs.
Wind gusts: The limiting factor for airliners1 (which rarely do the maneuver described above) is mostly due to wind gusts and up-/downdrafts. This can (for a short amount of time) result in large changes of the load factor, even into negative gs for a strong gust in the vertical direction. When experiencing strong turbulence, the airspeed should be reduced to turbulent air penetration speed (VB), which reduces the impact of the gusts.

Turbulent Air Penetration
Severe turbulence should be avoided if at all possible. However, if severe
  turbulence is encountered, use the Severe Turbulence procedure listed in the
  Supplementary Procedures chapter of the FCOM. Turbulent air penetration speeds
  provide high/low speed margins in severe turbulent air.

(Boeing 737 NG FCTM 1.50 General Information)

1 Limits for large aircraft from EASA CS-25:

(1) The positive limiting load factor
  must not be less than:
(i) 2.5 g with the EFCS
  functioning in its normal mode and with
  the high-lift devices retracted up to
  VMO/MMO. The positive limiting load
  factor may be gradually reduced down
  to 2.25 g above VMO/MMO.;
[...]
(2) The negative limiting load factor
  must be equal to or more negative than:
(i) -1.0 g with the EFCS
  functioning in its normal mode and with
  the high-lift devices retracted;

I would also add to this a clarification that the G limit is not for the "wing" as the question says, but for the entire aircraft. This is why limits such as Va (maneuvering speed) change with aircraft weight. It's not just the load on the wings (which decrease with weight for a given amount of lift, so in theory could accommodate more acceleration as weight goes down), but the load on every component and attachment point within the aircraft (which will experience the same force at a given G regardless of overall aircraft weight).

Five short, generic reasons (i.e., not specific to the Weedhopper):

Fatigue reduction: some highly stressed parts of the airframe (particularly mainspars and engine mounts) are susceptible to fatigue failure from cyclic loading. The key word here is 'cyclic'; a spar which is subjected to loads between (say) -5g/+5g will fail faster than one which is subjected to loads of only -1g/+5g. This can be demonstrated easily by bending a paperclip until it snaps: bending it fully one way and then fully the other will cause it to snap faster than simply bending it fully one way, then straightening it and bending again, etc.
Fuel systems: fully aerobatic aircraft typically have some form of pressurised fuel tank, fuel injection, or similar system to prevent fuel starvation under negative g. Case in point; Merlin engines in the early part of WW2 (Spitfires, Hurricanes, etc.) used a simple updraft carburettor. This would rich-cut under negative g as the fuel in the float bowl moved to the top of the carb, forcing the float to move downward and open the fuel valve all the way. This was fixed later in the war with the addition of pressure carbs, however early in the war this gave the German pilots a potential combat advantage: their fuel injected engines didn't have this flaw, meaning they could simply pull negative g to cause a pursuing allied aircraft to lose power.
Oil and coolant systems: related to point 2, if an engine has an oil pickup at the bottom of a sump (as most car oil systems do) then this can potentially be starved of oil under certain manoeuvres if the sump is improperly baffled. Aircraft intended for aerobatics may be fitted with a different oil system (such as a dry-sump system, which uses a separate oil tank and pressure pumps) to limit damage to the engines due to oil starvation.
Cargo storage: items (overnight bags, life jackets, fire extinguishers, etc.) stored loose under/behind seats may come free and move around the cockpit under negative g if they're not properly secured (admittedly this point involves some degree of common sense, although I've seen pilots at the local airfield who simply put their overnight bag behind the pilot's seat).
Airfoil profile: This point is very dependent on specific aircraft, but a negative g is often accompanied by a negative angle of attack of the wing. Depending on the aircraft in particular, this may lead to flow instabilities over the wing itself, and also the elevator section. This can result in buffeting of the stabilizer/elevator, or a loss of control authority of those surfaces, resulting in (worst case) a departure from controlled flight (stall/spin, etc.) or damage to/structural failure of the tail.

That is quite a good answer to a question about load factors and inverted flight. I don't think that was the question asked by  Jessica Ham.

@MikeBrass: the point about evading a pursing aircraft with negative Gs is at least a hint that this is by something other than rolling inverted and climbing, otherwise they could just follow without rolling.  Upvoted as a nice complement to other answers that point out that pushing into a dive can cause negative Gs.

Welcome new user!  This is a great post; you may have missed the thrust of the OP's question.

"...they could simply pull negative g to cause a pursuing allied aircraft to lose power."  I would like a better explanation of this tactic, and how it could be employed effectively.  Because if you are the pursuer and know you cannot sustain negative G you would simply roll 180 degrees and keep positive G.  And you would have a lot more G available than the guy who is negative, giving you the advantage.

The Merlin engine cut-out problem in the Spitfire was worked-around by a simple modification developed by female engineer Beatrice Shilling. It involved introducing a flow restrictor with a small orifice. Pilots were delighted by the immediate improvement in performance and allegedly, after a sortie, the Pilot's Mess would resound to toasts of "Miss Shilling's Orifice"!

First, I would like to point out the question was about limit load on airplane wings, which is different that load limits for the entire aircraft. Therefore, this sounds like an airframe question, not propulsion.
Let's look from the wing frame of reference: it does not care if aircraft is going level, up, down, sideways, or upside down. All it cares is whether the vertical component of the force is pointing up (positive) from its frame of reference or down (negative).
Non aerobatic wings tend to have non symmetrical profiles since they are optimized for flying right side up. This structural non-symmetry also leads to it being able to handle different loads in the positive vs negative directions.
Flying an aircraft is all about knowing what are the safe limits and staying inside them, be it the minimum takeoff speed for a given load, how much fuel you need to reach a given destination, how much weight you can have in the aircraft while landing, or how far you can push the wings before they snap or at least become rather fatigued.
Incidentally, a continuous maneuver at a given wing load is much less stressful to the wings/airframe than vibrating the same wing to the same wing load and back. Also, modern wings can handle wing load better because they distribute the load instead of concentrating it close to the root, which is why when an old style wing break off it is close to the fuselage.


text:
Why don't American passenger airlines operate dedicated cargo flights any more?
I was reading recently that Northwest Airlines was the last American passenger carrier to operate dedicated Cargo aircraft. Why do none of the large US airlines operate dedicated cargo flights anymore? This seems to be common among international Airlines (Lufthansa, China Airlines, Qatar Airways, EVA, etc).
Presumably UPS and Fedex's extensive networks have something to do with it? I'm surprised that none of them find it economical, however.

And Amazon now too.

Amazon doesn't have passengers, which makes them more like UPS or FedEx. Except they contract out (wet lease?) their planes for the most part, to Atlas Air, for example.

Alaska Airlines, Hawaiian Airlines still have dedicated freighters.
American, Delta, Southwest and United all have thriving cargo shipping operations, but as far as I can tell, no longer have freighters.

https://www.aacargo.com
https://www.alaskaair.com/content/cargo/general
https://www.deltacargo.com
https://www.hawaiianaircargo.com
https://www.swacargo.com

Alaska used to run 737 Combi to ship cargo and passengers in the main cabin on their “Milk Run” up the Alaskan panhandle. They now have dedicated freighters to handle the cargo.

https://thepointsguy.com/2017/10/final-flight-alaska-airlines-737-400-combi/

Good luck shipping your parcel.

The question is about cargo-only aircraft, not belly cargo operations. Alaska Airlines is a special case, they only service Alaskan routes with their 737F. Besides, the general public can't ship belly cargo post 9/11, you must be TSA-approved.

@user71659 And the answer explicitly says that Alaskan has cargo-only aircraft. You can claim that they're a special case but the question claims that no US passenger airline operates cargo-only flights and this answer rebuts that claim.

@DavidRicherby The real answer to the question is the economics of cargo compared to the geography of North America (you have to have a network). Alaska is a special case. The fact that combi/narrowbody cargo in Alaska is profitable is inapplicable to the lower 48.

@user71659 So what? Alaska is still part of the USA. Alaskan is still an American carrier. None of what you're saying invalidates the answer in any way.

Alaska and Hawaiian still have dedicated freighters. American does not have one, neither does Southwest or United . I can't tell if Delta still has freighters.

@user71659 "the real answer" has been covered by others. This supplemental answer provides good information which is completely on-topic and on-point.

@user33375 No it doesn't. It would be on-point if it said "While economics of mixed freight airlines on the mainland are uneconomic, the unique geography of Hawaii and Alaska creates demand for specialized operators". But it doesn't. No context. Alaska didn't even want to get the 737F, their combi was forced out of service due to FAA fuel tank interting requirements.

I would say that Hawaiian and Alaskan certainly count, and I had not considered looking them up when I originally thought of the question. Which is silly given I posted an answer about the Alaskan 737 Combis a few days ago :facepalm:

This answer wins because Alaskan flies cross-country (and cross-continent) and does indeed have dedicated 737-700 Freighters. But the other answers still provide a lot of valuable context as to why the largest US passenger airlines stopped operating dedicated cargo flights.

@user71659 "Your premise is wrong, X is not true" is a perfectly accurate answer to "why X?"  So is "X is not true technically; the reason why it is mostly true is Y".  So this answer would be improved by talking about why it is mostly true, but it remains an on-topic answer.

@zymhan While they fly cross-country, their freighters do not. Unless chartered, the freighters only fly within Alaska and to Seattle. That's my whole point that Alaska and Hawaii are special cases. Alaskan Airlines is really two different airlines in one: an airline serving rural Alaska, and a LCC serving the mainland.

A lot of airlines (USA flagged or not) still ship cargo in the hold along with passengers baggage, for some airlines its a high dollar business. Carriers like UPS, FedEx, etc, have surely put a dent in the plane-full-o-cargo market but for the airlines it actually helps to mitigate risk. If you carry both cargo and passengers you can be assured of a more stable revenue stream across the board. An airline can mitigate a lull in travel or cargo movement by also generating income from the other stream.
There is also a lot of differing logistics in moving cargo that a passenger airline may not want to deal with. UPS and FedEx also maintain truck fleets to deal with the package once it gets to an airport. This end to end business model is attractive to consumers. An airline, who may have the space on the plane but not the trucks, can't offer such service and for a given customer this may make or break the deal.

That makes sense, I noticed that Delta still lets you arrange cargo shipments on their scheduled passenger flights. It makes sense to have both in the same plane so you can make money from two different sources.

You'd also need warehouses/processing facilities for large-scale cargo operations, in order to get things on the right plane (and packed efficiently), then transferred to trucks, and have every package tracked through the process.

In the US? Because "trucks", +1, in a country with a federally maintained interstate highway system.

@Mazura Trucks are mostly last-mile haulage. The privately-maintained railroad system is the more direct competitor to air freight.

@chrylis: Rail isn't really a competitor to air freight.  Rail ships heavy, often bulk materials at low cost; air ships high-value materials where speed is important. (Imagine shipping grain or bulk chemicals by air :-))

Rail + Ship containerized transportation is a competitor to international air freight (you have 4 options from, for example, Shanghai to Rio de Janeiro (ignoring local haulage: 1: Ship through Panama (cheap / slow), 2: Rail to Europe, Ship across Atlantic (faster, but more expensive than #1, 3: Rail to Europe, Airfreight across Atlantic (still faster, more expensive than #2, includes need to change from ISO Containers to Air Containers), and 4: Air freight from China to Brazil (fastest, most expensive).

The cargo operation (dedicated fleet) requires its own logistical and operational apparatus.  Unless an airline's cargo subsidiary is large enough to get the required economy of scale, along with decent market conditions, it's not worth the trouble and expense.
According to this article, Lufthansa's cargo operation lost money in 2016 and they were complaining about subsidies to Gulf operators that allow them to undercut airlines like Lufthansa who have marginally profitable cargo divisions.
And there's the rub.  A good chunk of cargo operations outside the North America are subsidized (certainly the ones operated by government owned or controlled airlines).  In the absence of subsidies, and with a harder focus on making every dollar count, in North America it was found to be more efficient to specialize.

That makes a lot of sense, a cargo operation would require a lot of additional infrastructure that's not helpful for the passenger side of the house.

Razor...thin...margins.


text:
What equipment qualifies as FAA-acceptable for providing "Supplemental Oxygen"?
FAR 91.211 tells us when supplemental oxygen is required, but it's not very specific as to how that oxygen is to be provided. FAR 23.1447 gives some information about installed systems, but I can't find much on FAA requirements if you're using a portable system (cylinder, regulator, and cannula/mask like a Mountain High system).
Are there specific requirements/recommendations regarding portable oxygen equipment for Part 91 General Aviation, or can you just pick up a a medical or welding tank, stick a regulator on it, and pass out masks to your passengers?

I don't think a welding tank is fit for human respiration, though it seems a heavy debate on the internet

I don't have references right now so I'll save an answer for later, but you should have aviators oxygen because it has the water removed.  The others actually have enough water in them that they could freeze during a rapid decompression, which is just when you would need it the most.

@Lnafziger I was more thinking about the sort of equipment you'd carry on an unpressurized plane, but as far as the fill yes: it's important that the oxygen used in pressurized aircraft is "dry" - around here everything I can get my hands on (ABO, welding oxygen, and diving oxygen) is dry condensed O2 - they fill it all from the same LOX truck). I think medical oxygen is the same these days (at least in hospitals and local EMS the standard is to use an external water chamber to humidify it). Still not a big price difference to get ABO at the airport :)

@ratchetfreak the big incompatibility is often the regulator fitting (if you can't hook up your masks to the tank it's not gonna be too useful)

O2 in the US is normally sourced from the same fractional distillation plant. The difference between ABO, medical and welding that the purity requirement of O2 is greatest for welding.  The practical difference is that ABO and medical tanks are sampled after filling to assure contents.

An even more important question than how is how much oxygen to take, but there's no specific FAA guidance or regulation on either point. Different aircraft can carry different equipment, and people have different requirements depending on their age, lungs, blood chemistry, whether they smoke, day vs. night etc. The FAA's guide on oxygen equipment for GA use is extremely limited and although it describes a range of equipment types it doesn't mention any specific requirements.
Having said that, the 'usual' approach seems to be to use a pulse oximeter to measure your oxygen saturation and if it's too low then go on oxygen immediately, regardless of altitude or other considerations. The moderator of AOPA's medical forum is an AME and he recommends oxygen use any time your saturation level drops below 90%, which is apparently also the required level in hospitals for a patient to make an "informed" - i.e. clearheaded - decision.
But returning to the question of equipment, there aren't any regulations on this at all, so provided that the equipment is safe to use in the aircraft (including not obstructing exits and so on) and not illegal under any other law that might apply (carriage of dangerous goods or whatever) then you can go for it. But the usual 'always act as if you'll have to justify your decisions to the FAA tomorrow' advice applies here, IMO, and personally I would always go with an aviation-specific setup if I needed one.

I thought the FAA had something in an advisory circular on flow rates (LPM) at various altitudes for mask/cannula use - that would probably make another interesting question. Everyone I know just uses a pulse oximeter though - better than guesswork with flow rates and differing lung conditions/capacities.

It would also be important to SECURE the oxygen bottle so that it doesn't become a missile in an accident/turbulence, and to protect it so that the regulator isn't broken off.

If your aircraft is certified under Part 23 as opposed to CAR3, it would be obligated to abide by the following:
§23.1441   Oxygen equipment and supply.
(a) If certification with supplemental oxygen equipment is requested, or the airplane is approved for operations at or above altitudes where oxygen is required to be used by the operating rules, oxygen equipment must be provided that meets the requirements of this section and §§23.1443 through 23.1449. Portable oxygen equipment may be used to meet the requirements of this part if the portable equipment is shown to comply with the applicable requirements, is identified in the airplane type design, and its stowage provisions are found to be in compliance with the requirements of §23.561
I added the boldness to the text.
So, what portable oxygen is identified in an aircraft's type design? What is a "type design" anyway?
One source says Type design is this:
(1) The drawings and specifications, and a listing of those drawings and specifications, necessary to define the configuration and the design features of the product shown to comply with the requirements applicable to the product;
(2) Information on dimensions, materials, and processes necessary to define the structural strength of the product; and
(3) The Airworthiness Limitations section of the ICA as required by Parts 23, 25, 27, 29, 33, and 35, or as otherwise required by the Administrator
So how do you get from needing it to be identified in the type design to its ok to use it? Unless you find where it is identified... you can't connect them.
But CAR3 was more lenient on this and portable oxygen systems do not violate it. I find no mention of Oxygen requirements in CAR3.


text:
What United States state and federal agencies generally participate in aerial weather modification?
Also wondering which agencies regulate weather modification activities? Any idea where I can find more information about this?

The North Dakota State Water Commission runs the North Dakota Cloud Modification Project, which primarily seems to work at reducing hail damage to crops during the thunderstorm season, and has been active (perhaps not continuously) since 1975.
They've got a section on program evaluations; a peer-reviewed article published in 1997 states:

[...] the expanded 13-year NDCMP data base showed a 45% decrease in losses in the target area (Smith, et al., 1997).

Their website has a ton of information; a company called Weather Modification, Inc. is the contractor for ND State and does a lot of other work as well. Either the state or WMI would probably be able to provide additional data.
It may also be worth noting that Russia does lots of weather modification work as well (perhaps more when it was the Soviet Union). No idea how successful any of it is.

From Wikipedia

In the US, the National Oceanic and Atmospheric Administration keeps records of weather modification projects on behalf of the Secretary of Commerce, under authority of Public Law 92-205, 15 USC § 330B, enacted in 1971

Thank you, Do you have any information about which agencies engage in weather modification?

No agencies do it successfully!

there is a company called EverGreen Air that was contracted out to do some flying and spraying. I used to see a lot of activity over Sheridan, Wy around 2001 -2004 but I do not see it that much any more and really never see it were I moved to. The mountains that ran from outside Billings, Montana to Denver, Co I think were a big experimentation ground for this type of activity.


text:
When and how can pilots waive wake turbulence holds in the US?
According to FAA rules, controllers are required to hold aircraft 2 or 3 minutes for wake turbulence under certain conditions. When is that wake turbulence hold required, and when are pilots allowed to waive it? What is the correct way to request a waiver?

This on takeoff, I assume. I would also wonder when it is safe to waive the hold.

The AIM 7-3-9 lists the required separations. Behind a heavy aircraft, the departure hold is 3 minutes and it may not be waived. Behind a small aircraft, the hold may be waived on pilot request only:

3. Additionally, appropriate time or distance intervals are provided to departing aircraft:
(a) Two minutes or the appropriate 4 or 5 mile radar separation when
  takeoff behind a heavy/B757 jet will be:
(1) From the same threshold.
(2) On a crossing runway and projected flight paths will cross.
(3) From the threshold of a parallel runway when staggered ahead of
  that of the adjacent runway by less than 500 feet and when the runways
  are separated by less than 2,500 feet.
NOTE- Controllers may not reduce or waive these intervals.
b. A 3-minute interval will be provided when a small aircraft will
  takeoff:
1. From an intersection on the same runway (same or opposite direction) behind a departing large aircraft,
2. In the opposite direction on the same runway behind a large aircraft takeoff or low/missed approach.
NOTE- This 3-minute interval may be waived upon specific pilot
  request.
c. A 3-minute interval will be provided for all aircraft taking off
  when the operations are as described in subparagraph b1 and 2 above,
  the preceding aircraft is a heavy/B757 jet, and the operations are on
  either the same runway or parallel runways separated by less than
  2,500 feet. Controllers may not reduce or waive this interval.

EDIT: The AIM 4-3-10(f) (intersection takeoffs) also addresses this question, including suggested pilot phraseology for waiving the hold (emphasis mine):

Controllers are required to separate small aircraft (12,500 pounds or
  less, maximum certificated takeoff weight) departing (same or opposite
  direction) from an intersection behind a large nonheavy aircraft on
  the same runway, by ensuring that at least a 3-minute interval exists
  between the time the preceding large aircraft has taken off and the
  succeeding small aircraft begins takeoff roll. To inform the pilot of
  the required 3-minute hold, the controller will state, “Hold for wake
  turbulence.” If after considering wake turbulence hazards, the pilot
  feels that a lesser time interval is appropriate, the pilot may
  request a waiver to the 3-minute interval. To initiate such a request,
  simply say “Request waiver to 3-minute interval,” or a similar
  statement. Controllers may then issue a takeoff clearance if other
  traffic permits, since the pilot has accepted the responsibility for
  wake turbulence separation.

Wake turbulence in general is considered a key safety issue by the FAA and there is a large amount of information in the AIM, the ATC guide and elsewhere.

As far as required separation goes, they are changing (only at some airports for now) so take a look at: http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2012/SAFO12007.pdf


text:
What is the FMS Bridge Visual 28R approach at SFO?
I've read that SFO has a specialized FMS Bridge Visual approach to 28R that is not published but instead is custom tailored for specific operators. What exactly is this approach and how does it work? When is it assigned, and what advantage does it have over other visual approaches to runway 28R?

The FMS Bridge Visual Approach 28R is a version of the Quiet Bridge Visual Approach 28R which is coded with GPS coordinates and can be included in an FMS database for approved operators.  This allows the procedure to be used when the SFO
VOR is out of service, and also gives ATC additional flexibility by allowing them to clear pilots direct to any of the fixes without needing to intercept the radial on the standard arrival.
The pilot must either tell ATC that they can accept it verbally, or include FMS28R ABLE in the remarks of their flight plan so that they know that the flight is capable of flying the approach.

Source: stilllearningtofly.org

The FMS bridge visual is an RNAV charted visual approach to 28R that keeps aircraft offset to the north of the 28R localizer until they’re near the San Mateo bridge. This allows a little bit more space between aircraft that are pairing up on final. Procedurally it’s no different than the tipp toe visual for 28R/L. Charted visuals allow the controllers to clear an aircraft for the approach once they have a charted landmark or the airport in sight. Vs a standard visual where the pilot must have the airport in sight. We will automatically assign the FMS to most airliner traffic inbound from the East over ARCHI. GA or some foreign carriers will get the quiet bridge visual or the ILS.

I tried to upload a bigger image but it said it was too big. This is all I could get to work. I’m on my phone too, which might be the problem.

Thanks for adding a better picture! I’m not a pilot, so I’m not sure how the company approaches work. I’ve heard that only the airlines that buy it are able to do the FMS. I’m not sure if United designed it and sold it to others. Most major airlines have it, it’s not exclusive to 1. But foreign / smaller airlines and GA will ask for the quiet bridge.

Murph - welcome to Av.SE!  Always good to have controllers join us here to share your perspectives on various topics.  Please hang around!

Ralph J-  Thanks!  I didn’t know I let it slip that I was a controllers.  This is a cool place to ask questions and get answers!


text:
What are the US definition and restrictions on aerobatic flight?
In the US, what is the legal definition of aerobatic flight? What are the restrictions (both for the aircraft and its location/maneuvers) applied to aerobatic flight? I remember reading something about a restriction on degrees of pitch or bank, but I was looking around for it and can't seem to find it again.

Aerobatic flight is "an intentional maneuver involving an abrupt change in an aircraft's attitude, an abnormal attitude, or abnormal acceleration, not necessary for normal flight."
According to the FAA (91.303), no person may operate an aircraft in aerobatic flight:

Over any congested area of a city, town, or settlement;
Over an open air assembly of persons;
Within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport;
Within 4 nautical miles of the center line of any Federal airway;
Below an altitude of 1,500 feet above the surface; or
When flight visibility is less than 3 statute miles.

Aerobatic flight is sometimes considered to be flight with more than 30 degrees pitch or more than 60 degrees bank. This is incorrect. It applies to wearing a parachute. (FAR 91.307)
A normal category aircraft is certified for the following "non-acrobatic" operation: Stalls, lazy eights, chandelles, and steep turns, in which the angle of bank is not more than 60°.
A utility category aircraft is certified for spins (if approved for plane), lazy eights, chandelles, and steep turns, or similar manoeuvres, in which the angle of bank is more than 60 degrees but not more than 90 degrees.
An acrobatic category aircraft is required for other aerobatics.

Is it possible that a utility category aircraft MUST be approved for those listed maneuvers, but MAY be approved for others as well such as a loop?


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What is the range and accuracy of ATC radar systems?
What is the average range, accuracy, and update frequency of radar systems used to monitor airspace in the USA?
I'm primarily interested in capabilities for tracking smaller general aviation aircraft without enhanced return data channels like ADS-B, etc.
Please include sources, if possible. As an example, by looking at TRACON/ASR data exports (CDR), these systems appear to update an aircraft echo at about a 4.7 second frequency, give or take a few tenths. However, I not been been able to source or verify my findings or similar statements made by expert witnesses in litigation cases.

I almost wonder if this is more suitable for answering in a foreshortened format, such as focusing on one kind / class of radar system

Also, I fear you are drifting too close to national security topics.  Not that the information should be secret, but that it may be voluntarily non-disclosed.  I am guessing, and hope to be wrong

RE: Scope of question. I suppose depends on how many kinds/classes of radar systems there are in use. I believe the list (at a high level) is very short though.
RE: national security. I wondered if this would come up myself. I think it could get questionable if someone were able to provide highly technical details on specific radar systems. However, I think at a high level, this question is asking for knowledge that should be considered public knowledge and might be useful to aviators and those, like me, that are interested aviation accident analysis.

These are too many questions into one. Consider breaking it up. If you ask what is the difference between primary and secondary radar, I am happy to answers, and also if you are interested in difference between Mode A, C and S. But all combined with specific radar types it becomes a bit too much. The basic question in the title I can answer though

You may get better traction, even just insofar as 'best questions' by posting info about the radar systems you know about, and asking for their differences.

I have edited my question to limit scope per comments. I was originally hoping for a comprehensive answer that would answer the original question, and by doing so, inherently answer all the related subquestions. Per feedback though, perhaps that was expecting too much due to the nature of radar systems.

The smaller general aviation aircraft you mention, do they have a Mode A(/C) transponder (ATCRBS)?

I am primarily interested in raw radar capabilities assuming no return channel, so no Mode A(/C), ATCRBS, ADS-B, etc.

There are sources online claiming that the terminal Airport Surveillance Radar used by the FAA rotates at 12.5 RPM (one update every 4.8 sec) and the enroute Air Route Surveillance Radar rotates at 5.0 RPM (one update every 12 sec). I have not been able to find official documentation for these figures.
In an article titled "FAA Surveillance Radar Data as a Complement to the WSR-88D Network" Mark Weber includes the following unsourced table:

TDWR (Raytheon)
ASR-9 (Northrop Grumman)
ASR-11 (Raytheon)
ARSR-4 (Northrop Grumman)

Transmitter

Frequency
5.5 - 5.65 GHz ~ C Band
2.7-2.9 GHz
2.7-2.9 GHz
1.2-1.4 GHz

Polarization
Linear
Linear or Circular
Linear or Circular
Linear or Circular

Peak Power
250 KW
1.1 MW
20 kW
60 kW

Pulse Width
1.1 μs
1.0 μs
1.0 μs, 80 μs
150 μs

PRF
2000 (max)
2 CPIs (~ 1000 Hz avg.)
4 CPIs (~ 1000 Hz avg.)
9-pulse CPI at variable spacing (288 Hz avg)

Receiver

Sensitivity
0 dBz @ 190 km 1m2 @460km
0 dBz @ 20 km 1m2 @111km
0 dBz @ 20 km 1m2 @111km*
0 dBz @ 10 km 1m2 @370km

Antenna

Elevation Beamwidth
0.55 Degrees (min)
5 Degrees
5 Degrees
2 Degrees (stacked)

Azimuth Beamwidth
0.55 Degrees
1.4 Degrees
1.4 Degrees
1.4 Degrees

Power Gain
50 dB
34 dB
34 dB
35 dB (transmit), 40 dB (receive)

Rotation Rate
5 RPM (max)
12.5 RPM
12.5 RPM
5.0 RPM

He does not provide citations for this information but there is no reason to doubt it.
The FAA's ASR-11 Test and Evaluation Master Plan document does not mention rotation rate (that I can see) but does list some detailed "Critical System Characteristics" beginning on page 16, for example:

PSR Probablity of Detection (Pd)
The PSR shall be capable of detecting a 1.0 m2 Radar Cross Section (RCS), Swerling 1 target to a range of 55 nmi in the clear and at the nose of the beam with a single scan Pd of 0.8 and a Probability of False Alarm (PFA) of 10-6.

This is a general answer, not specific applicable to the situation in the USA.
There are several types of radars in use to monitor airspace.
Basically you can distinguish two classes of radar:

Primary radar does not require the aircraft to carry a transponder, it is a form of non-cooperative surveillance.
Secondary radar on the other hand requires the aircraft to carry a transponder; the transponder replies to interrogations from the radar. This is called cooperative surveillance.

Radars operate basically by line-of-sight, however due to atmospheric effects the radio waves follow the curvature of the earth to some degree. Aircraft flying low are quickly below the horizon, but aircraft at FL450 can be detected on secondary radar at ranges up to 300NM if the radar is operated for such range. Typically the range is less.
In general, the further the range that a radar needs to look, the slower it rotates. An airport surface radar may rotate as fast as twice per second, long range en-route radars can go as slow as once per 12 seconds. For typical TMA / TRACON use, 5 seconds would be in the right ballpark, for en-route 8 seconds will be about right.

The accuracy depends on the type of antenna, whether it is primary or secondary radar, and the distance of the aircraft from the radar head. The accuracy of the distance measurement are not so much affected by the range, and varies from about 5 meters to 300 meters. However, atmospheric circumstances can degrade these figures, especially at long range. The across beam accuracy is very much dependent on the distance from the antenna due to limited angular accuracy of the radar. The further you get, the worse the across beam accuracy becomes.

I originally posted this simply as a comment, but I suppose it's more of a partial answer.
I researched this about six months ago and found that, in the US, 6 seconds was the standard for a TRACON, and 12 seconds for an ARTCC - allowing them to see about a 60nmi and 120nmi radius, respectively. I don't have sources off-hand, but if I come across them, I'll post them.
Not sure about the accuracy, but when it's a human looking at a screen, discrepancies of even tens of feet is probably not particularly meaningful.

As a side note, we definitely need to get some controllers engaging in this site.

I'll try and convince some to join

Thanks. This answer is closer to my findings. In all fairness, I already have some specific data regarding ATC radar capabilities. However, I've never been able to source it or verify it beyond statements by expert witnesses in litigation cases, and by watching actual radar echos in recorded data. As an example, according to CDR data exports, TRACON/ASR systems appear to update an aircraft echo at about a 4.7 second frequency, give or take a few tenths. I'll update my question with this note. And I agree, having ATC controllers engaged on this site would be beneficial to everyone.


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What are common airline crew titles and positions?
I realize that airlines can operate with different numbers of Pilots and Flight Attendants, depending on the route, aircraft and length of flight (EET). Which are the types of crew members that an airline can have, with the correct titles? I've heard about captains, master captains and cruise captains, but how can I identify them during the flights?

As the EET is variable from flight to flight you should better say that it is dependent on specific kinds of operation or routes, which is quite the same but includes also the time before and after the flight. Flight duty time starts at check in and ends at check out.
There are general requirements based on the maximum allowed duty time of a crew member, which are again based on the regulations of the aviation authority of the operators country. Finally it differs from airline to airline and is regulated by the operations manual, which can of course be not less restrictive than the authority, but there are some general titles you often hear:

Captain The Captain is the PIC (pilot in command) of this flight and responsible for a safe operation. He must be on station for departure and arrival.
First Officer The F/O is also found on every airliner operation. On short or medium ranges he is the 'Copilot', some airlines fly with two F/Os on long haul flights and in some portions of this flight most probably the two F/Os are in the cockpit and the captain takes a nap.
Second Officer When starting at an airline, you sometimes begin as a S/O and are only alloweed to do specific tasks. So for example handling the radios during departure and arrival, or having the controls during the cruise phase.
Cruise Captain Well, read the name ;) He might be a Captain and only in the function of a Cruise Captain for a specific flight, but once again, it depends on the operator.

Stubing was the most famous cruise captain of them all.... oh right.

You miss the identification part (the number of strips).


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What are the requirements to get your private pilot certificate?
I am 15 right now and I want to get the Certificate/License when i am 17. (17 is the age right?)
How much training do I need and is there any written tests etc?
What kind of IFR training will be included/needed?

In the US, you'll need to be 16 old for solo flights in training and 17 to get your Private Pilot's license in a piston-powered aircraft. (You can begin training at any age, but must be 16 to solo.) To get your license you'll have to have a minimum of 40 hours total time, including at least 20 with an instructor and 10 solo. You will also have to pass a written test, and get a Medical certificate.
Details:
If you're pursuing a Private Pilot license, per 14 CFR §61.83(a) or (b) you must be 16 years old to get a student pilot certificate (or 14 years old if you're going to train in a balloon or glider.) An SPC is required for solo flight, but not for dual instruction.
NOTE: 14 CFR mean Code of Federal Regulations, Title 14 - Aeronautics & Space.  §61.## means "Part 61" which covers Certification of Pilots and Instructors.
To start training you will also have to pass a medical exam for a 3rd Class Medical Certificate, per 14 CFR §61.23(a)(3). If you're generally healthy, getting a 3rd Class Medical isn't terribly difficult.
Assuming you successfully complete the training, per 14 CFR §61.103(a) or (b) you can get your Private Pilot license for an airplane at 17 years old (or 16 years old for a balloon or glider.)
Per 14 CFR §61.109(a) to get your license you will need to have completed at least 40 hours of flight time, including at least 20 hours of time with an instructor and 10 hours of solo time.
However, many pilots take more than the required minimum 40 hours to get their license. My suggestion is to try to complete your training in as short a calendar time as is possible. By taking lessons frequently (more than once per week) you will minimize the amount of "repeat" lessons, which will help you get your license with fewer hours. If you're only taking a lesson every few weeks, you will end up repeating topics and spending more than the required 40 hour minimum.
Per 14 CFR §61.35, you will have to complete a ground school program (can be with an instructor, or a "home study" course) before you take the Knowledge test (aka: written exam.)
Per 14 CFR §61.109(a)(3), you will need at least 3 hours of "Instrument" flight training to get your Private Pilot license. That's really not intended to be enough for you to fly safely on instruments, but to give you a basic set of skills to get out of a low-visibility situation. If you keep up with your flying, getting in Instrument rating is a GREAT idea, once you have enough hours.
You can look up the specific wording of all of the rules at http://www.ecfr.gov/
Another good resource for student pilots is the Aircraft Owner's and Pilot's Association's Learn to Fly page http://www.aopa.org/Pilot-Resources/Learn-to-Fly.aspx.

Note that the requirement is 16 to solo an airplane - you may begin your flight training at any time, the student pilot certificate and medical (where applicable) are not required until you are ready to solo.

Ok. I was gonna say that if you have to start training at 16, I've been training when I was 13 so I was beginning to think i was doing somehting Illegal. Both Answers help!

Also, is the ground training like for Taxiing and stuff like that?

@CameronTarbell "Ground school" is the classroom aspect of flight training, it covers a lot of things, including basic aerodynamics, operating rules & regulations (the FARs), flight planning, airport signs/markings, and - if your ground instructor is good - practical things like risk management. You will also cover most or all of these things in some way as part of the flight portion of your training, but that tends to be focused on building skills, where ground school is focused on building knowledge.

One thing to note is everything in user264's answer is the minimum required and solely lies upon the individual to:

Study - yes flying an airplane does require some academics, basic math, navigation, basic physics. The more of the basic reading you do up front will help very bit with keeping hours down on learning to fly
Your personal willingness to be humbled - flight is an art - you will hear it at some point, every pilot judges their flight by the landing. The reason for this is it is the hardest part of flying. All the training you receive will be things to teach you about landing. Do well with those and they will help you to make good landings. It will be a challenge but well worth it
Listen and ask lots of questions - your instructor is trying to lead an art class.
On average it takes 50-70 hours to make a private pilot. Usually closer to 70

Younger folks seem to pick it up sooner - better reflexes, eyesight, things along those lines.


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How precise is an ILS (Instrument landing system)?
I read that an ILS is a precision approach system. It provides a final approach with precision in both horizontal and vertical guidance to the runways. But I don't know how precise it is because there were a number of accidents related to landing operations.

I would assume that it is extremely accurate, as autolanding things are able to land it pretty much perfectly every time

The ILS is precise enough to allow landings in essentially zero visibility, including those performed by autoland.
The ground based bits
The vertical range is 1.4° (0.7° above glideslope, 0.7° below) and the horizontal varies, but is a maximum of 6° wide. What this means in terms of how many feet or meters off the centerline an aircraft is varies depending on where the aircraft is on the approach, but it's generally correct to say that the closer an aircraft gets to the ILS antennae, the more precise the system is.
The specifications for a localizer antenna require that it be no less precise than ±10.5m - this is for a Cat I approach, and it gets correspondingly more precise as the category increases.
There's always the risk of interference with the transmitters; as noted in the comments, accidents have been caused by airport vehicles parked in the ILS critical area, interfering with the signals, and as the equipment ages it can require recalibration.
The airborne bits
It's possible for receivers to have issues that could affect precision; antennas can get loose connections or become covered in ice, while pilots might interpret indications incorrectly, misread reverse sensing, or intercept a 'false glideslope' from above.
Regarding accidents
You're correct - there are many accidents related to landing operations. That said, you may be including those which weren't flying an ILS approach - it's hard to say since you aren't citing anything. Still, it's possible that there's confusion between navigation precision and safety of flight.
Many (not all) of landing accidents involving instrument approaches do center around human error rather than equipment failure; pilots tuning in the wrong frequency or incorrectly setting avionics and instruments (reversing the inbound course on an HSI causes reverse sensing, that sort of thing), or people on the ground screwing up.

There's also the issue with false lobes.

@roe true, but I'm not sure if that falls into the 'precision' category ... technically, the false lobes are just as precise as the correct ones. They're just... wrong. Inaccurate yet precise, if you will.

Precisely inaccurate? :) I don't know if there are any accidents attributed to false lobes, but it could potentially, with all its precision, guide you down in reverse sensing steep glide slope, causing confusion resulting in loss of control for example.

Also, although the question says "precision", I think it is fair to say the answer "it's precise, just not pointing you where you want to go" is splitting a few hairs... :)

The major issue with ILS accuracy (or for that matter precision) is there's no way to measure it from the aircraft: The ILS gives you two needles (Fly Up / Fly Down & Fly Left / Fly Right), but it has no way of telling you if the signal has been perturbed by say a truck parked next to the ILS antennas (which has been known to happen). Signal attenuation from that sort of thing mainly affects accuracy, but could conceivably affect precision as well.

@egid Unless you look at it from an entire system perspective. Since he asks in relation to landing accidents I think that we need to include the receiver too as well as the external factors that are part of the overall system.

ILS is can be very precise, but there are many factors that may compromise the precision. When all these factors are avoided, automatic landing with no visibility is reliably possible.
The ILS installations are classified in three categories:

CAT I: Category 1 allows for decision height (DH) no less than 200 ft (height, so above ground level).
ILS creates the signal by interference of different signals emitted by offset antennas. If one of them fails, but not the other, the signal may appear as perfectly centred to the instrument on board of the approaching aircraft with no way to tell that it does not actually work. Therefore pilots should cross-reference their position using other sources (DME, VOR, ADF, marker beacons).
There is a video about this kind of failure (describes an actual incident where the glide slope stopped working without indication, the crew noticed by cross-checking against DME and altitude, plus description of the failure), but I can't find it now.
CAT II: Category 2 allows for DH not less than 100 ft and allows using auto-land until touch-down. It needs to have far field monitoring that can detect incorrect or incomplete signal and the transmission system has to be doubled. It also needs to be somewhat more precisely aligned than CAT I.
CAT III: Category 3 allows auto-land including roll-out. It has subcategories IIIa that allows DH not less than 50 ft, IIIb that allows DH down to 0 ft (so effectively no DH) and visibility 75 m (otherwise the plane could not taxi off the runway as there is no guidance for taxiing; when there is, it will be called CAT IIIc). It has stricter limits on precision and monitoring than the CAT II.

Independent of the category, direct line of sight is needed. The signal is not affected by fog and precipitation, but is distorted by any obstacles. Aircraft or even small vehicle crossing the runway or taxiing in front of the transmitter will significantly distort the signal.
That's why each ILS CAT II and CAT III installation has ILS protected areas and to actually use auto-landing, special procedures need to be in effect that keep these areas clear. When they are not, incidents like Singapore B773 at Munich on Nov 3rd 2011, runway excursion happen.
Similarly attempting to auto-land on CAT I lead to accidents like Air India A320 at Jaipur on Jan 5th 2014, runway excursion on emergency landing.

Quoting egid:

"The vertical range is 1.4° (0.7° above glideslope, 0.7° below) and
  the horizontal varies, but is a maximum of 6° wide. What this means in
  terms of how many feet or meters off the centerline an aircraft is
  varies depending on where the aircraft is on the approach, but it's
  generally correct to say that the closer an aircraft gets to the ILS
  antennae, the more precise the system is."

I know of at least one reader who has a problem with this statement, so I'm only adding these comments for clarification.  EGID is correct.  Think of the localizer as a funnel ... the "hopper" end of the funnel catches more liquid when it is bigger and fans out wider ... but it ultimately leads to the same narrow downspout.  If the purpose of this funnel is to catch and direct more incoming liquid (aircraft) then it's doing no one favors to have too narrow of a hopper.  All funnels are very precise on the small end.  You want an ILS to be fairly forgiving for the reason that you may be intercepting it while blind and in the soup.

The localizer is placed at the far end of the runway (if it was at the approach end, it would become very erratic close to touchdown, and we do not want that), hence it cannot diverge more than to cover the runway at the other end, or you'd be coming down on a taxiway, or park it directly in a hangar. This doesn't really matter if your needle is both accurate and precise and you're able to keep it centered, I just wanted to point out that the touchdown point is not at the 'hopper' end of the funnel of the localizer (glide slope is another matter though), it's actually a couple of miles away.

@roe: The main reason why localizer is at the far end is that it still needs to indicate correctly while you are over and on the runway.

@JanHudec; yes, that's what my first sentence said (check the parenthesis) :) The rest still holds, the touchdown point is nowhere near the end of the funnel, and if the precision isn't good enough, you'll end up off the runway, so it isn't all "the same narrow downspout"

@roe: I checked the parenthesis. But I am saying it's not it would become erratic, it would flat out not be available at all.

@JanHudec; You're right, it would be unavailable for a short while over the antennas before capturing on the back course (being reverse sensing at that point). Don't know for how long it would be silent, but it'd probably be available again before reaching the touchdown zone, no? Either way, it's a bad idea, and completely beside the point I was trying to make. :)


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What is a "core lock" on an jet-engine?
This question came up while reading the Wikipedia article on core lock and Pinnacle Flight 3701.
If my understanding is right then if for example inner parts of the jet engine heats up that so much that the metal will expand, then after that it cools down rapidly it can result in a lock of the ball bearing of the blades. This means the engine is not functional anymore?
The Wikipedia article says that a windmill and a APU-restart is not possible anymore - is this because the engine has been rendered permanently inoperable?
Could somebody explain exactly what a core lock is to me, possibly with pictures?

There is a good writeup here that explains the engine issues as well as the rest of the incident you mentioned.
Core lock happens when parts inside the engine cool off at different rates, causing interference between them. In the case of the CF34 engines on these Bombardier jets:

GE isolated the problem to contact between components of the high-pressure-turbine interstage seal - a static, pressurized honeycomb component - and the rotating seal teeth on the outer torque coupling.

Working on getting a good illustration of what they are talking about.
This does not mean that the engine is "not functional." There is just much more resistance on the rotating components, but once the temperatures have time to equalize, the components will break free.

According to the NTSB, FDR data indicate that the engine cores were beginning to break free just before the impact.

During core lock, the resistance makes starting the engine more difficult. A windmill start does not provide very much turning force on the engine components, so the resistance from core lock makes that method of starting difficult. GE has done engine testing, and when the windmill restart fails, they are able to use bleed air from the other engine to restart the engine, which provides more turning force.

Thanks for this really great and professional answer! This writeup is excellent, thank you!

While starting core-locked engine is possible with sufficient starter pressure, it will still cause significant wear and thus reduce it's expected life.

My experience shows that, on some short haul domestic type operations, some pilots practice shutting down the left wing-mounted engine quickly after landing to be sure the engine has stopped turning to allow passengers to disembark down the steps. The quick turnaround operation may mean another engine start within 30 minutes of shut down. The core (heavy turbine discs) is still hot whereas the lighter weight and exterior turbine cases have cooled and contracted onto the turbine airseals. This can lead to "seal squeal" if the engine is windmilling in the wind and attracts ground engineers' attention, or in severe cases may lock the core engine shaft. In most cases, allowing the engine to fully cool or, if possible, restarting the engine, allow 10 mins at idle to fully expand all parts equally and recut the turbine airseals, the engine can be deemed serviceable if all other parameters are normal. Continued use by pilots of this practice would mean for the engine to quickly lose performance, mostly seen as increased fuel consumption, through premature airseal deterioration.